1. Field of the Invention
This invention relates to a steel-making process in a converter, and more particularly to a steel-making process wherein carbon monoxide, which is cheap and effective for improving the service life of tuyere without damaging the quality of molten steel, is used as a blowing gas for effectively agitating molten steel in a refining vessel to advantageously promote the refining reaction.
2. Related Art Statement
In general, an oxygen top-blown converter (LD process) has been used for obtaining molten steel be decarburizing hot metal and simultaneously reducing impurity elements such as P, S and the like in hot metal.
In this type of the converter, a bottom blowing process with an inert gas has been recently popularized for the enhancement of refining properties.
It is known that the bottom blowing of the inert gas is largely effective for strengthening the agitation of molten steel bath in the converter as well as for the reduction of impurities, increase of iron yield, increase of control accuracy between component and temperature at blowing end and the like.
Heretofore, argon (Ar), nitrogen (N.sub.2) or carbon dioxide (CO.sub.2) has been used as an agitating gas.
Ar gas is completely inert to molten steel, so that it is adaptable for the above mentioned object but is expensive.
On the other hand, N.sub.2 gas is cheaper than Ar gas but dissolves into molten steel, so that the nitrogen concentration in molten steel is increased during the blowing in the converter, which may raise problems in the quality of steel.
CO.sub.2 gas is able to reduce an amount of top-blown O.sub.2 gas by a part corresponding to a decarburization reaction of CO.sub.2 +C.fwdarw.2CO produced when the carbon concentration in molten steel is high, and is relatively low in the cost, so that the use of CO.sub.2 gas is economically advantageous. However, CO.sub.2 gas is an oxidizing gas different from Ar and N.sub.2 gases, so that the service life of a tuyere for blowing the gas into molten steel or a porous plug becomes shorter.
Besides the above gases, it has bee proposed to use an unburned exhaust gas recovered from the converter or a so-called LD gas as a cheap agitating gas. However, such an LD gas contains a fairly great amount of N.sub.2 gas and CO.sub.2 gas, which comes into problems in view of the service life of tuyere and the increase of nitrogen concentration in molten steel as mentioned above, so that the use of LD gas is not yet practised.